PROGRESSIVE EFFECTS, AND CONTINUED ACTION OF CAUSES.
Sometimes a complex effect results, not (as has been supposed in the
last four chapters) from several, but from _one_ law. The following is
the way.
Some effects are instantaneous (e.g. some sensations), and are
prolonged only by the prolongation of the causes; others are in their
own nature permanent. In some cases of the latter class, the original is
also the proximate cause (e.g. Exposure to moist air is both the
original and the proximate cause of iron rust). But in others of the
same class, the permanency of the effect is only the permanency of a
series of changes. Thus, e.g. in cases of Motion, the original force is
only the _remote_ cause of any link (after the very first) in the
series; and the motion immediately preceding it, being itself a compound
of the original force and any retarding agent, is its _proximate_ cause.
When the original cause is permanent as well as the effect (e.g. Suppose
a continuance of the iron's exposure to moist air), we get a progressive
series of effects arising from the cause's accumulating influence; and
the sum of these effects amounts exactly to what a number of
successively introduced similar causes would have produced. Such cases
fall under the head of Composition of Causes, with this peculiarity,
that, as the causes (to regard them as plural) do not come into play all
at once, the effect at each instant is the sum of the effects only of
the then acting causes, and the result will appear as an ascending
series. Each addition in such case takes place according to a fixed law
(equal quantities in equal times); and therefore it can be computed
deductively. Even when, as is sometimes the case, a cause is at once
permanent and progressive (as, e.g. the sun, by its position becoming
more vertical, increases the heat in summer) so that the quantities
added are unequal, the effect is still progressive, resulting from its
cause's continuance and progressiveness combined.
In _all_ cases whatever of progressive effects, the succession not
merely between the cause and the effect, but also between the first and
latter stages of the effect, is uniform. Hence, from the invariable
sequence of two terms (e.g. Spring and Summer) in a series going through
any continued and uniform process of variation, we do not presume that
one is the cause and the others the effect, but rather that the whole
series is an effect.